An image sensor converts a visual image to digital data that may be represented by a picture. The image sensor comprises an array of pixel sensor cells, which are unit devices for the conversion of the visual image into the digital data. Digital cameras and optical imaging devices employ an image sensor. Image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While CMOS image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
A CMOS image sensor unit cell comprises a pixel area and a logic area. The pixel area typically contains one of each type of active pixel, e.g., a red pixel having a first photodiode underneath a red optical filter, a green pixel having a second photodiode underneath a green optical filter, and a blue pixel having a third photodiode underneath a blue optical filter. The pixel area may also contain a dark pixel, which is covered with an opaque layer such as an aluminum light shield and employed to measure a background output level in the absence of illumination so that the output of other pixels may be referenced and calibrated. U.S. Pat. No. 6,750,912 to Tennant et al., which is incorporated herein by reference, describes the use of a dark pixel. Each photodiode generates charges upon exposure to light. Logic devices connected to the photodiode detect and amplify the charges to generate signals proportional to the incident light. Each pixel comprises at least one photodiode to covert incident photons into electrical charges.
During a read out of the charge from a photodiode, charge carriers stored in a charge collection well is transferred through a body of a transfer transistor to a floating drain. The transfer of the charge needs to be complete to maximize the signal strength from the pixel. Electrical charges remaining in the photodiode after the transfer of the charge through the transfer transistor causes “image lag” when the next image is taken by the photodiode. A photodiode that retains residual electrical charges from the prior exposure due to the incomplete transfer of the electrical charges thus imparts some of the image from the previous exposure to the image from the next exposure. Thus, the next image includes a component of the previous image through the image lag.
The image lag causes signal loss for a current image. Further, the image lag mixes the signals from the current exposure with the signals from the previous exposure, which is partially retained in the photodiode through the residual electrical charges in the photodiode after the incomplete charge transfer during the previous operation. The trapping of the residual charges in the photodiode is typically caused by a potential barrier between the charge collection well and the channel of the transfer transistor, which prevents transfer of all of the electrical charges during a read operation or a reset operation.
In view of the above, there exists a need for a CMOS image sensor structure that provides reduction of image lag. Particularly, there exists a need for a CMOS image sensor, that effects a complete transfer of electrical charges from the photodiode to the floating drain as quickly as possible, i.e., before the next image is taken.